The present embodiments relate to wireless communications systems and are more particularly directed to such a system including a transmitter implementing a variable intermediate frequency in its upconverter.
Wireless communications have become prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. One such advancement includes the use of spread spectrum communications, including that of code division multiple access (“CDMA”). In such communications, a user station (e.g., a hand held cellular phone) communicates with a base station, where typically the base station corresponds to a “cell.” CDMA systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. CDMA continues to advance along with corresponding standards that have brought forth a next generation wideband CDMA (“WCDMA”) and which has a 3GPP standard.
Also with the prevalence and advancement of wireless devices, commercial competition proceeds at a considerably rapid pace. In the competitive marketplace, considerations are made in numerous aspects of devices as well as communication standards and protocols. Additionally, consumer demands and expectations are heavily considered. As a result, factors such as incremental costs, device size, reliability, and longevity are all important in the development of wireless devices.
Given the preceding, the present art includes a transmitter device that includes various functional blocks, one of which is typically referred to as an up-converter. By way of further background, FIG. 1 illustrates an electrical block diagram of such a prior art up-converter designated generally at 10, and which is typically included as part of the functional circuitry of a transmitter. Up-converter 10 includes a reference frequency generator 11 that provides a reference frequency to a first phase-locked-loop (“PLL”) voltage-controlled oscillator (“VCO”) 12 and also to a second PLL VCO 14. First PLL VCO 12 outputs a fixed intermediate frequency (“IF1”) to an input of a quadrature generator 16. In the prior art where up-converter 10 is used in a WCDMA design, IF1 is set to 380 MHz. The output of quadrature generator 16 is connected to mixers 181 and 182, each of which receives a respective baseband input I and Q. Quadrature generator 16 produces two signals at the same frequency, namely, at the frequency of IF1, where those two signals are separated from one another by a ninety degree phase shift. These two signals are connected to respective mixers 181 and 182, thereby adjusting the baseband frequency of each of I and Q based on IF1. The outputs of mixers 181 and 182 are combined and then connected to an input of a variable gain amplifier 20, which has its output connected to an input of a first surface acoustic wave (“SAW”) filter 22 as further discussed below.
With respect to the connection to the input of SAW filter 22, it is noted that FIG. 1 also includes a dashed line DL1 encompassing various of the blocks of up-converter 10. Dashed line DL1 is intended to illustrate the boundaries of an integrated circuit used to implement up-converter 10 in the prior art, that is, in the prior art, all blocks within dashed line DL1 are included in a single integrated circuit. Thus, returning to SAW filter 22, note now that it is external from the single integrated circuit represented by dashed line DL1. Accordingly, the above-discussed connection to the input of SAW filter 22 requires an external connection from the integrated circuit bounded by dashed line DL1. The output of SAW filter 22 is connected as an input to a mixer 24 that is within the integrated circuit represented by dashed line DL1. Thus, this connection also requires an external connection with respect to the integrated circuit.
Returning to blocks within the above-introduced integrated circuit, mixer 24 receives at another input a local oscillator signal (“LO1”) from second PLL VCO 14. In the prior art, LO1 is variable so that it may be selected based on one of various different WCDMA transmission channels, where the different channels are from a set of channels in a transmission band spanning 1922.4 through 1977.4 MHz. Specifically, LO1 is chosen so that the sum of frequencies provided by IF1 and LO1 is equal to the final transmission channel frequency. More particularly, in WCDMA, each transmission channel is 3.84 MHz wide (i.e., it has a 3.84 MHz bandwidth), and each WCDMA transmitter is operable to transmit along any of these channels. Typically, a given transmitter operates to transmit along one of these channels according to the cell in which the transmitter is located, while the transmitter adjusts to transmit along a different channel when the transmitter is re-located to a different cell. The channels are selected from the WCDMA transmission band spanning 1922.4 through 1977.4 MHz. For example, while in a first cell, the transmitter may transmit information along a final transmission channel frequency of 1922.4 MHz; in this case, LO1 is set to provide such an output. Particularly, assuming low side frequency injection (i.e., LO1<final transmission frequency) by mixer 24, then at this time second PLL VCO 14 outputs LO1 at a frequency of 1542.4 MHz so that it is combined with the IF1 frequency of 380 MHz to provide a final transmission channel frequency of 1922.4 MHz (i.e., IF1+LO1=380 MHz+1542.4 MHz=1922.4 MHz). As another example, while in a second cell, the transmitter may transmit information along a final transmission channel frequency of 1932.4 MHz; in this case, and again assuming low side frequency injection by mixer 24, then at this time second PLL VCO 14 outputs LO1 at a frequency of 1552.4 MHz so that it is combined with the IF1 frequency of 380 MHz to provide a final transmission channel frequency of 1932.4 MHz (i.e., IF1+LO1=380 MHz+1552.4 MHz=1932.4 MHz). Given these examples, one skilled in the art will recognize that LO1 will range, for low side injection in WCDMA, from 1542.4 MHz to 1597.4 MHz. Thus, for any instance in this range, LO1 is provided to mixer 24, thereby adding the frequency of LO1 to the IF1 frequency of 380 MHz, and the output is connected as an input to an image reject filter 26. Image reject filter 26 is less complex than SAW filter 22 and, as a result, it is feasibly integrated within dashed line DL1. The output of image reject filter 26 is connected as an input to a driver 28, and the output of driver 28 is output from dashed line DL1 and, thus, is connected externally from the integrated circuit represented by dashed line DL1. More specifically, the output of driver 28 is connected as an input to a second SAW filter 30. The output of second SAW filter 30 is connected as an input to a power amplifier 32. The output of power amplifier 32 is connected to an antenna ATU1. Lastly, although not shown, up-converter 10 is typically not only part of a transmitter, but that transmitter is usually accompanied by a receiver circuit or circuitry as well. In this regard, the output of power amplifier 32 is also typically connected to a duplex circuit and returned to the receiver circuit so that the signals for transmission may be suppressed with respect to the receiver functionality so as not to interfere with the receiver that is part of the same overall device.
The operation of up-converter 10 is generally as follows. Quadrature generator 16 provides appropriate phase shifted signals, and having a frequency equal to IF1, to mixers 181 and 182. Mixers 181 and 182 also receive the baseband signals I and Q. As a result, the frequency of each pair of signals into a mixer are summed to add the 380 MHz IF1 frequency to the respective baseband signal; thus, in up-converter 10 this provides a first increase in the frequency of the baseband signal. The result is amplified by variable gain amplifier 20, and note that the variability of that amplifier allows adjustments, for example, for reasons such as the distance between a wireless base station and the unit that includes up-converter 10. Due to the first frequency multiplication of mixers 181 and 182 and also due to the amplification from amplifier 20, various spurious frequencies are introduced into the resulting signal. Indeed, it is recognized that as the signal is further processed through up-converter 10 and its frequency is modified further for final transmission, these spurious signals can negatively affect other devices operating in both the WCDMA band as well as in other wireless bands (e.g., UMTS, EGSM, GSM, DCS, Bluetooth, and GPS). As a result, SAW filter 22 is provided so as to reduce such spurious frequencies. In other words, SAW filter 22 reduces or removes any harmonic frequencies of the 380 MHz IF1 frequency (i.e., integer multiplies of 380 MHz for integers greater than one). In addition, SAW filter 22 reduces noise from the signal. Since the output of SAW filter 22 is connected to mixer 24, then its frequency is again adjusted, this time based on LO1; thus, in up-converter 10 this provides a second increase in the frequency of the baseband signal. Recall that in FIG. 1 this adjustment is a low side injection, that is, the LO1 frequency is less than the final transmission channel frequency and the LO1 frequency is summed with IF1 to reach the final transmission channel frequency (i.e., the final transmission channel frequency is provided by LO1+IF1). As known in the art, however, the multiplication by mixer 24 necessarily also produces the difference of these two frequencies, namely, |LO1−IF1|. Image reject filter 26 therefore removes this differential frequency and, thus, allows only the summed frequency to pass. Alternatively, if up-converter 10 implemented high side injection (i.e., LO1>final transmission channel frequency), then image reject filter 26 removes the summed frequency and allows the differential frequency to pass. For either the high side or low side injection case, after filter 26 the signal is amplified by driver 28 to a value sufficient to drive the input requirements of power amplifier 32. Before reaching power amplifier 32, second SAW filter 30 reduces any remaining spurious frequencies and noise from the signal after which it is amplified by power amplifier 32 and transmitted via antenna ATU1.
While up-converter 10 of FIG. 1 has proven acceptable in the past, the present inventors have observed that it has various drawbacks. For example, recall that up-converter 10 includes both an integrated circuit that performs a portion of the up-converter functionality as well as various external devices coupled to the integrated circuit to complete the up-converter functionality. In the example of FIG. 1, the external devices include at least two discrete filters, namely, SAW filters 22 and 30, which are commercially available from numerous sources. The present inventors have observed that the use of these SAW filters, while functionally sufficient, may provide various drawbacks. For example, each externally required SAW filter increases, beyond the integrated circuit, the overall space and weight required to implement the up-converter functionality. As another and perhaps more critical example, each SAW filter adds to the overall cost to implement the up-converter functionality, and any incremental cost in the extremely competitive marketplace that currently exists can be critical as to the viability of the device that implements the up-converter functionality. Still other drawbacks may be ascertained by one skilled in the art.
In view of the above, there arises a need to address the drawbacks of the prior art as is achieved by the preferred embodiments described below.